(a) Field of the Invention
The present invention relates to a microscope objective and, more particularly, to a microscope objective with the magnification about 20X having a sufficiently large aperture as required, i.e., N.A. about 0.4, and still having a long working distance and flat image surface.
(b) Description of the Prior Art
As microscope objectives with flat image surface, it is known that there are such lens systems in which a meniscus lens having a concave surface with a small radius of curvature on the object side is arranged as the first lens component, and lens systems in which a meniscus lens concave toward the image side is arranged in a rear position. The objective disclosed in Japanese published unexamined patent application No. 45741/72 which adoptes the Gauss type lens configuration is known as a lens system having a still longer working distance. However, the objective disclosed in said patent application has a five-component eight-element lens configuration and, when the working distance is made long in case of a lens system with a number of lens elements as mentioned in the above, rays cannot be converged unless the refractive power of the first lens component is made strong. As a result, the radius of curvature of the first lens component becomes small and spherical aberration increases. When it is attempted to correct spherical aberration, the function for correction of aberrations should be alloted to other lenses and, as a result, it becomes difficult to correct curvature of field, coma, etc. Due to the reason described in the above, there is no known microscope objective of which the working distance is long and, at the same time, the image surface is flat over a sufficiently wide field.